As this kind of combustion plate, there is conventionally known in Patent Document 1 a combustion plate in which flame holes are formed over the entire surface of the combustion plate such that three kinds of large, middle, and small flame holes are positioned so that: various kinds of flame holes are distributed in lattice shape; and that the large hole is positioned in the center of the four adjoining small flame holes and is also positioned in the center of the adjoining four middle flame holes; that each of the small flame holes is formed so as to be positioned in the middle of the adjoining two middle flame holes; and that on the surface of a plate main body there is formed a bottomed hole which is coaxial with each of the large flame holes and partly includes each of the small flame holes that are present in the circumference of the large flame hole. It is said therein that, according to the above-mentioned arrangement, the combustion resonant sounds and instability at the time of high-load combustion that are likely to occur in an arrangement in which the flame holes are all made to be of the same diameter, can be dissolved.
By the way, in the Patent Document 1 a description is made of a combustion plate, in the combustion plate of which the diameter of the large flame hole is made to be 1.9 mm, the diameter of the middle flame hole is made to be 1.3 mm, the diameter of the small flame hole is made to be 1.0 mm, and also four small flame holes are disposed on the circumference of 2.4 mm in diameter that is coaxial with the large flame hole, and four middle flame holes are disposed at an equal distance to one another, in a phase deviated from the small flame holes by 45 degrees, on the circumference of 3.4 mm in diameter that is coaxial with the large flame hole.
However, in the art described in the Patent Document 1, due to the fact that flame holes of different diameters are disposed in lattice shape, the opening ratio (the ratio of total area of the entire flame holes to the total area of the combustion region of the plate main body) becomes comparatively small. In the example described above, the opening ratio was about 26%. Therefore, there was a disadvantage in that a passage resistance through the combustion plate becomes large, with a resultant increased load on the fan to supply primary air to the burner increase, whereby the fan noises become large.